Avenger Strike Fighter

The Bastard offspring of the Warthog and the Stuka.

The Avenger Strike Fighter is an Imperial Navy close-range ground-attack aircraft meant for destroying armor and other heavy targets. It does this with the Avenger Bolt Cannon, the S6 AP-2 D1 Heavy 8 gun around which the flyer was built, as well as 2 lascannons and 2 hardpoints that can mount anti-infantry or anti-vehicle weapons. Since it was supposedly used a lot by Sebastian Thor's forces during the Age of Apostasy, the Sisters of Battle see it as an instrument of divine vengeance (through superior firepower), so they use it a lot. The Avenger model is produced by Forge World, released to coincide with Imperial Armour Aeronautica (the book containing the rules for it and other flyers, what with Warhammer 40,000 6th edition having flyer rules).

Forge World was very obviously inspired by the real-world A-10 Thunderbolt II (or "Warthog"), a ground-attack aircraft built around the tremendously powerful GAU-8/A Avenger Gatling cannon. It shoots 30mm depleted uranium shells so fast that it doesn't go "Dakka," it goes BRRRRRRT. Despite this shared characteristic, the Avenger's overall shape is more akin to a Ju-87 Stuka: both have inverted gull wings but the ones on the Stuka are more triangular. It is a ground-attack plane after all.

Like the modern A-10's cannon, the Avenger's Gatling cannon isn't all that great against battle tanks and similar vehicles because 40k game mechanics don't use side armour to resolve hits from flyers like they do hits from barrage weapons (unless you are a chad and play 30k, where you can penetrate from the side or the rear). Just like modern tanks, almost all 40k tanks (except siege tanks, like the Vindicator) would be side armour (AV10-12) on the top, and therefore quite vulnerable to the hail of S6 shots. That said, with the Strafing Run and Tank Hunter rules (the latter available only in the Horus Heresy version), its Gatling cannon is actually quite effective against AV 10 or 11 targets. Taking antiarmor hard points, though, allows you some tactical flexibility as you can use the big gun for mowing infantry while the hard points take out armor.

In 8th edition with the removal of AV, its namesake main gun struggles against most vehicles for a different reason. Unlike its big cousin bolt cannons the Castigator and Vulcan, with which it shares the same Strength and AP, its bolts do 1 damage each rather than 2. This seems to shift its use away from being an anti-vehicle weapon that can target infantry (like autocannons) and toward being an anti-infantry weapon that can target vehicles (like assault cannons). Its relation to the assault cannon is kind of like that of the heavy bolter to the big shoota: similar weapon, but firing bolts instead of bullets for improved penetration, though the Avenger has better range and fire rate. But if we focus on what the main gun can do rather than what it should do, it does have impressive utility. T7, 8 and 12+ are bad targets for it, but with it's AP -2 pretty much everything else is fair game. Ideally, you'd want to point it at dangerous armoured infantry, and on a flying platform that often means strafing their devastator/dark reaper equivalents. It's also good for bypassing necron quantum shielding, and decent at shaving the last wound needed to kill or cripple a vehicle that just ate the Avenger's lascannon fire. Barring that, you just point it at whatever few of your other units are able to take out. That can mean T9/10 models that lascannons will struggle against, light vehicles that more specialized weapons don't want to bother with, infantry in an inconvenient place, and more besides.
Forces of the Imperial Guard
Command: Commissar - Enginseer - Imperial Guard Command Squad - Ministorum Priest
Primaris Psyker - Regimental Advisors - Tank Commander
Troops: Armoured Fist Squad - Infantry Squad - Field Chiurgeon - Heavy Weapons Squad
Militarum Veteran Squad - Ogryn Squad - Penal Legion - Psyker Battle Squad
Ratling Squad - Rough Rider Squad - Special Weapons Squad - Stormtrooper
Transports: Aurox Armoured Transport - Argoran Armoured Transport - Chimera
Crassus Armored Assault Transport - Gorgon Armored Assault Transport
Hades Breaching Drill - Hellbore - Mole - Taurox - Termite - Testudo
Trojan Support Vehicle - Pegasus AAV
Light Vehicles: Atlas Recovery Tank - Bike Squad - Bane Wolf - Centaur Utility Vehicle
Cyclops Demolition Vehicle - Devil Dog - Hellhound - Land Crawler - Scylla
Salamander Reconnaissance Tank - Sentinel - Siegfried - Tauros - Venator
Pegasus AFV
Tanks & Ordnance: Basilisk Artillery Gun - Carnodon - Colossus Bombard - Deathstrike Missile Launcher
Griffon Heavy Mortar Carrier - Heavy Quad-Launcher - Hydra Flak Tank
Leman Russ Battle Tank - Manticore Launcher Tank - Medusa Siege Gun
Ragnarok - Wyvern Suppression Tank
Superheavy Vehicles: Baneblade - Capitol Imperialis - Leviathan - Macharius Heavy Tank
Malcador Heavy Tank
Flyers & Bombers: Avenger Strike Fighter - Lightning Fighter - Marauder Bomber
Thunderbolt Fighter - Valkyrie - Vendetta - Vulture
Forces of the Sisters of Battle
Command: Canoness - Ecclesiarchy Battle Conclave
Ministorum Priest - Sororitas Command Squad
Troops: Avenging Angel - Arco-flagellant - Battle Sister Squad
Celestians - Crusaders - Death Cult Assassin
Dominion Squad - Retributor Squad - Seraphim Squad
- Sisters Repentia
Vehicles: Exorcist - Immolator - Penitent Engine - Repressor - Rhino
Flyers: Avenger Strike Fighter - Drop Pod
Saints: Living Saint - Geminae Superia
Non Militant Orders Dialogous - Orders Famulous - Orders Hospitaler
Forces of the Adeptus Mechanicus
Command: Magos (Tech Priest Dominus)
Troops: Combat Servitors - Electro Priest - Enginseer - Kataphron Battle Servitors
Myrmidon - Secutarii - Skitarii - Tech Priest - Tech Thrall - Thallax - Ursarax
Chrono-Gladiator - Servo-Automata
Legio
Cybernetica:
Castellan-class robot - Cataphract-class robot - Colossus-class robot
Conqueror-class robot - Crusader-class robot - Scyllax-class robot
Thanatar-class robot - Vultarax stratos-automata
Walkers: Ironstrider Ballistarius - Onager Dunecrawler - Sydonian Dragoon
Vehicles: Chimera - Galvanic Servohauler - Karacnos Assault Tank - Krios Battle Tank
Land Raider - Macrocarid Explorator - Minotaur Artillery Tank - Ordinatus
Rhino Transport - Triaros Armoured Conveyer - Skorpius Hover Tank
Flyers: Avenger Strike Fighter - Lightning Fighter - Storm Eagle
Titans: Imperator Battle Titan - Imperial Knight - Reaver Battle Titan
Warbringer Nemesis Titan - Warhound Scout Titan - Warlord Battle Titan
Vehicles of the Imperium of Man
Walkers Dreadnought - Nemesis Dreadknight - Ironstrider Ballistarius
Onager Dunecrawler - Penitent Engine - Sentinel - Sydonian Dragoon
Transports Chimera - Crassus Armored Assault Transport - Drop Pod - Goliath Truck
Gorgon Armored Assault Transport - Hades Breaching Drill - Immolator
Razorback Transport - Repressor - Rhino Transport - Taurox - Testudo - Pegasus AAV
Trojan Support Vehicle - Triaros Armoured Conveyer - Tunneling Transport Vehicles
Light
Vehicles
Atlas Recovery Tank - Bane Wolf - Bike Squad - Centaur Utility Vehicle - Scylla Light Tank
Cyclops Demolition Vehicle - Devil Dog - Hellhound - Land Crawler - Achilles Ridgerunner
Salamander Reconnaissance Tank - Siegfried - Tauros - Venator - Wolfquad - Pegasus AFV
Battle
Tanks
Caladius Grav-Tank - Krios Battle Tank - Land Raider
Leman Russ Battle Tank - Predator - Ragnarok - Sabre Tank Hunter
Sicaran Battle Tank - Spartan Assault Tank - Vindicator
Ordnance Basilisk Artillery Gun - Colossus Bombard - Deathstrike Missile Launcher
Exorcist - Griffon Heavy Mortar Carrier - Hunter - Hydra Flak Tank
Manticore Launcher Tank - Medusa Siege Gun - Rapier Armoured Carrier
Stalker - Thunderfire Cannon - Whirlwind - Wyvern Suppression Tank
Superheavy
Tanks
Baneblade - Capitol Imperialis - Cerberus Heavy Tank Destroyer
Fellblade - Leviathan - Macharius Heavy Tank - Macrocarid Explorator
Malcador Heavy Tank - Mastodon - Ordinatus - Typhon Heavy Siege Tank
Skimmers Javelin Attack Speeder - Grav-Rhino - Land Speeder - Pallas Grav-Attack
Flyers Caestus Assault Ram - Corvus Blackstar - Fire Raptor - Nephilim Jetfighter - Sky Talon
Space Marine Landing Craft - Storm Eagle - Stormbird - Stormhawk - Stormraven
Stormtalon - Stormwolf - Thunderhawk - Valkyrie - Vendetta - Vulture
Fighters &
Bombers
Avenger Strike Fighter - Lightning Fighter - Marauder Bomber
Stormfang - Thunderbolt Fighter - Xiphon Interceptor
Spacecraft Aquila Lander - Arvus Lighter - Boarding Torpedo - Devourer Dropship
Faustus Interceptor - Fury Interceptor - Shark Assault Boat - Starhawk Bomber
Tetrarch Heavy Lander
Titans Imperial Knight - Warhound Scout Titan - Reaver Battle Titan
Warlord Battle Titan - Warbringer Nemesis Titan - Emperor Battle Titan